Radar is often used to determine the relative velocity of a target with respect to a fixed or moving radar antenna. One technique used to measure the relative closing velocity of a target analyzes the Doppler shift that the target's velocity produces in the frequency of a reflected radar signal. If the target is approaching the radar antenna, its relative closing velocity causes an apparent decrease in the reflected radar signal wavelength, thus Doppler shifting the reflected signal to a higher frequency. Conversely, if the target's velocity is directed away from the radar antenna, the Doppler shift decreases the frequency of the reflected radar signal.
CW LIDAR is used in a manner analogous to radar, but in a more microscopic sense, to measure relative wind velocity as a function of a Doppler shift in the frequency of coherent light reflected from aerosol particles in an airstream. If the aerosol particles and the air in which they entrained have a relative velocity toward the source of coherent light, the light reflected from the aerosol particles experiences an increase in frequency, and conversely, a decrease in frequency with regard to air/particles moving away from the source of the coherent light with a relative velocity. However, there are several problems with conventional LIDAR systems that limit their usefulness and practical application in determining airstream velocity. Since the coherent light is simultaneously reflected from a plurality of aerosol particles moving through a relatively large volume of space, large fluctuations in signal intensity occur at the detector used to monitor the reflected light, due to interference between the reflected light signals. This problem is known as "speckle".
A more significant limitation in the performance of prior art CW LIDAR systems concerns their poor signal-to-noise ratio (SNR). As CW LIDAR is normally implemented to monitor airstream velocity, the SNR is independent of the optical aperture or diameter of the lens used to collimate the coherent light and also independent of the range from the lens to the aerosol targets, because any decrease in signal strength with range is offset by a corresponding increase in the number of aerosol particles that scatter or reflect the coherent light within the sample volume. For example, assuming that there are sufficient aerosol particles in the sample space to produce a generally constant optical backscatter coefficient, .beta., of 1.times.10.sup.-7, a 1 micrometer wavelength laser having a 1 watt power rating would have an SNR defined by: EQU SNR=(.eta..multidot.2.pi..multidot..lambda..sup.2 .multidot..beta..multidot.P.sub.t)/(h.multidot.c.multidot.B)(1)
where:
.eta.=0.05 (QE.cndot.T.sup.2 efficiency factor) PA1 2.pi.=6.28 PA1 .lambda.=10.sup.-6 PA1 .beta.=10.sup.-7 m.sup.-1 sr.sup.-1 PA1 P.sub.1 =1 watt PA1 h=6.6.times.10.sup.-34 Js PA1 c=3.times.10.sup.8 m/s PA1 B=2.times.10.sup.5 Hz (bandwidth)
Using the above values, an SNR of 0.8 is obtained, which is too low for use in a practical system. To achieve any usable results, prior art CW LIDAR systems have used higher power CO.sub.2 lasers. It should be apparent that laser diodes and diode pumped YAG lasers would be a preferable light source for many applications of CW LIDAR systems because of the ruggedness, small size, and potential low cost of these devices; however, laser diodes and diode-pumped lasers are generally not yet available at CW power levels above a few hundred milliwatts and, thus, can not be used as a source of coherent light in CW LIDAR systems of the prior art design.
Accordingly, a CW LIDAR system having much higher SNR is required for practical use in measuring the relative velocity of air or other fluids containing particles. Such a system and corresponding method for measuring the relative velocity of a fluid should be low in cost and capable of using a low-power coherent light source. These and other objects and advantages of the present invention will be apparent from the attached drawings and from the Description of the Preferred Embodiments that follows.